The present invention relates generally to wireless communications systems and, in particular, to extending access ranges of wireless communications systems.
FIG. 1 depicts a wireless communications system 10 employing Code Division Multiple Access (CDMA) techniques based on the well-known IS-95 standard of the Telecommunication Industrial Association. The wireless communications system 10 comprises a mobile switching center (MSC) 12 and a plurality of base stations (BS) 14-i connected to the MSC 12. Each of BS 14-i provides wireless communications services to mobile-telephones (MT), such as mobile-telephones 16-k, within an associated geographical coverage area referred to herein as cell 18-i with a radius Ri. For illustrative purposes, cells 18-i are depicted as circular in shape with base stations 14-i centrally positioned. It should be understood that cells 18-i may also be non-circular in shape (e.g., hexagonal) with the base stations positioned non-centrally, and that the term xe2x80x9cradius Rixe2x80x9d should be construed to define a distance between the base station and a point on the circumference of cell 18-i (which will vary depending on the particular point on the circumference).
Each base station 14-i includes radios and antennas for modulating and transmitting base station signals to mobile-telephones, and for receiving and demodulating mobile-telephone signals from mobile-telephones within its associated cell 18-i. Each base station 14-i further includes a receiver for receiving timing information using the well-known Global Positioning Satellites (hereinafter referred as a xe2x80x9cGPS receiverxe2x80x9d).
Signals are transmitted by base stations 14-i and mobile-telephones in accordance with a timing protocol aligned with GPS time using the GPS receiver. FIG. 2 depicts a timing schedule 20 incorporating an implementation of a timing protocol based on the IS-95 standard. The timing schedule 20 comprises a series of frames 22-n, wherein each frame 22-n spans a time interval t. The beginning of each frame 22-n is marked by a frame boundary at time Tn aligned to GPS time. In accordance with the timing protocol, base stations 14-i are configured to begin transmitting base station signals at the frame boundaries, wherein the base station signals include zero or more information bearing signals and a pilot signal for coherent demodulation of the information bearing signals by the mobile-telephones and system access operations. By contrast, mobile-telephones 16-k are configured to begin transmitting mobile-telephones signals at some multiple x of a frame time period (i.e., tx) after mobile-telephones 16-k began receiving base station signals, where x is some integer greater than or equal to zero. Unlike base station signals, mobile-telephone signals include one or more information bearing signals and no pilot signal, and are encoded using a set of orthogonal codes (referred to as Walsh codes) combined with a pseudo-noise (PN) sequence (or a known code) such that the information bearing signal may be non-coherently demodulated. The PN sequence comprises random 0 and 1 digital signals, wherein the duration for a 0 or 1 to transmit is referred to herein as a PN chip.
The above described timing protocol will now be discussed in reference to FIG. 3, which depicts a time chart 28 illustrating a sequence of transmissions and receptions by base station 14-i and mobile-telephone 16-k. At time T1, BS 14-i begins transmitting base station signal S1 to MT 16-k, which may be located anywhere in cell 18-i. MT 16-k begins receiving signal S1 at time T1+dBSxe2x86x92MT, where dBSxe2x86x92MT is a propagation delay from BS 14-i to MT 16-k. Note that the term propagation delay should be construed to include line-of-sight and non-line-of-sight propagation delays.
MT 16-k will wait a time interval tx from when MT 16-k began receiving signal S1 before it begins transmitting mobile-telephone signal S2. Thus, MT 16-k will begin transmitting signal S2 at time T1+dBSxe2x86x92MT+tx (or time dBSxe2x86x92MT after some frame boundary). For example, if x=2, then MT 16-k transmits signal S2 at time T3+dBSxe2x86x92MT (or two frames after receiving the base station signal S1).
Due to a propagation delay dMTxe2x86x92BS from MT 16-k to BS 14-i, BS 14-i will begin receiving signal S2 at time T1+dBSxe2x86x92MT+tx+dMTxe2x86x92BS. For ease of discussion, it is assumed that the propagation delay dMTxe2x86x92BS from MT 16-k to BS 14-i is the same as the propagation delay dBSxe2x86x92MT, and both will hereinafter be referred to individually as a one way propagation delay dow, i.e., dow=DMTxe2x86x92BS=dBSxe2x86x92MT, or collectively as a round trip propagation delay 2dow. Thus, BS 14-i will begin receiving signal S2 at time T1+tx+2dow.
In order to demodulate the received signal S2, BS 14-i must first detect signal S2. Each radio includes a correlator, which is a device that detects mobile-telephone signals. For example, the correlator detects mobile-telephone signal S2 by multiplying an incoming signal by the PN sequence, where the PN sequence is time shifted in discrete steps over a period or time interval (referred to herein as a search window Wn) until the resulting product (of the PN sequence and the incoming signal) exceeds a threshold indicating the detection of mobile-telephone signal S2. If BS 14-i does not begin to receive signal S2 within the confines of a search window Wn, BS 14-i will not be able to detect signal S2 (using the timing protocol incorporated in FIG. 2).
To ensure that BS 14-i begins receiving signal S2 within the confines of search windows Wn, search windows Wn should span time intervals that include possible arrival times for signal S2 (traveling a straight line or line-of-sight path between the mobile-telephone and the base station) regardless of the position of mobile-telephone 16-k in cell 18-i. Based on the above described timing protocol, base station 14-i can expect to receive signal S2 no earlier than the frame boundary and no later than time 2dow-radius after the frame boundary, where dow-radius is the one way propagation delay (or 2dow-radius is the round trip propagation delay) for a signal traveling a distance equal to the radius Ri. Thus, search windows Wn should span a duration of at least 2 dow-radius beginning at time Tn and ending no earlier than time Tn+2 dow-radius. In effect, the duration of search windows Wn restricts the effective radius (or size) of cell 18-i, which is also referred to herein as the access range of a base station.
The duration of search windows Wn depends on the implementation of the correlator. Typically, correlators are implemented in the form of an Application Specific Integrated Circuit (hereinafter referred to as an xe2x80x9cASIC correlatorxe2x80x9d) having a predetermined number of bits (also referred to herein as a xe2x80x9cbit limitationxe2x80x9d) for representing a round trip delay (of a signal traveling from the base station to the mobile-telephone and back to the base station). Such bit limitation limits the duration of the search windows which, as discussed above, limits the effective size of cell 18-i or access range of the base station 14-i. As long as the bit limitation does not limit search windows Wn to a duration of less than 2 dow-radius, base station 14-i should be able to detect signal S2 transmitted by any mobile-telephone located anywhere within its cell 18-i (assuming that Ri is the same for all points on the circumference).
Typical implementations of base stations in an IS-95 based CDMA wireless communications system include an ASIC correlator having a 12-bit limitation for representing the round trip delay. In order to have fine resolution of delay, a typical value of 1/8 PN chip is used as the minimum resolution unit. The 12-bit limitation (or round trip delay representation) in units of 1/8 PN chips yields a range of 512 PN chips (i.e., 212 bitsxc3x971/8 PN chips/bits). For a transmit bandwidth of 1.2288 MHz (which is typical for an IS-95 based CDMA wireless communications system), the 12-bit limitation can represent a round trip delay of 416 xcexcs (i.e., 512 PN chips÷1.2288 PN chips/xcexcs). With air propagation speed of 5.33 xcexcs/mile, the 416 xcexcs round trip delay (or 208 xcexcs one way delay) represents the limitation that if a mobile-telephone is located approximately 39 miles (i.e., 208 xcexcs÷5.33 xcexcs/mile) from the base station, the mobile-telephone is capable of communicating with the base station if the radio path loss is acceptable and the search window is configured correctlyxe2x80x94that is, the 12-bit limitation (or 512 time chip delay index representation) allows for a cell with a maximum radius Ri (or a maximum round trip delay) of approximately 39 miles. A signal transmitted by a mobile-telephone beyond 39 miles of BS 14-i, in accordance with the prior art timing protocol, may not arrive at BS 14-i within the confines of any search windows Wn and, thus, will not be reliably detectable with the 12-bit ASIC correlator.
Presently, if the cell size or access range is to be extended beyond the 12-bit limitation of the ASIC correlator (i.e., beyond 39 miles), the ASIC correlator would have to be re-designed. Specifically, the ASIC correlator would have to be re-designed to increase its bit limitation such that signals transmitted by mobile-telephones positioned beyond the access range 12-bit limitation of the ASIC correlator may also be detected. ASIC correlator re-design, however, is undesirable and may not be economical for small scale of applications. Therefore, there exist a need to extend the cell size or access range of the base station without incurring the high costs associated with ASIC correlator re-design.
The present invention is an extended range concentric cell base station and a method for extending a cell size or access range without incurring ASIC correlator re-design. This is accomplished with a concentric cell base station design that incorporates multiple timing protocols and search windows. The concentric base station has associated a micro cell and a macro cell, wherein the micro and macro cells use a same frequency band but different timing protocols and search windows that will cause signals transmitted by mobile-telephones within their respective cells to be received within the confines of at least one search window. In one embodiment, the micro cell uses the timing protocol of the prior art with a first search window that begins at the frame boundary and ends at some time p1 after the frame boundary, wherein p1 represents a time interval corresponding to a bit limitation of an ASIC correlator being used to represent the first search window. By contrast, the macro cell uses a modified timing protocol and a second search window that begins after the frame boundary but no later than the time p1 after the frame boundary (i.e., no later than the end of the first search window) and ends at some time p2 after the second search window began, wherein the modified timing protocol will cause the signals transmitted by mobile-telephones in the macro cell to be received within the confines of the second search window and p2 represents a time interval corresponding to a bit limitation of an ASIC correlator being used to represent the second search window.